About thirty years ago it became apparent that household laundry detergents made of branched alkylbenzene sulfonates were gradually polluting rivers and lakes. Solution of the problem led to the manufacture of detergents made of linear alkylbenzene sulfonates (LABS), which were found to biodegrade more rapidly than the branched variety. Today, detergents made of LABS are manufactured world-wide.
LABS are manufactured from linear alkylbenzenes (LAB). The petrochemical industry produces LAB by dehydrogenating linear paraffins to linear olefins and then alkylating benzene with the linear olefins in the presence of HF. This is the industry's standard process. Over the last decade, environmental concerns over HF have increased, leading to a search for substitute processes employing catalysts other than HF that are equivalent or superior to the standard process. Solid alkylation catalysts, for example, are the subject of vigorous, ongoing research.
To date, alkylation processes that use catalysts other than HF, that is, commercially available solid alkylation catalysts, tend to operate at a higher molar ratio of benzene per olefin than processes that employ HF. As an illustration, while detergent alkylation processes that use HF tend to operate at a benzene/olefin molar ratio of 12:1 to 6:1, alkylation processes that use commercially available solid alkylation catalysts tend to run at higher benzene/olefin ratios, typically 30:1 to 20:1. One reason for this is that solid alkylation catalysts tend to be less selective toward producing monoalkylbenzene, and therefore the benzene/olefin molar ratio must be increased to meet increasingly stringent selectivity requirements. Selectivity, which is often defined as the weight ratio of monoalkylbenzene product to all products, is expected in some areas to be 85-90% near term, increasing to 90-95% by about the year 2000. Incidentally, a higher benzene/olefin ratio not only tends to increase selectivity but also often produces other benefits for solid alkylation catalysts, including improving olefin conversion, monoalkylbenzene linearity, and catalyst life.
One configuration of an HF detergent alkylation unit that gained wide acceptance during the 1970's and 1980's uses an HF alkylation reaction section, an HF regeneration section, and an HF sludge treatment section. It is not necessary here to describe these three sections in detail, but one of the compelling reasons for switching from HF to a solid catalyst is that building and operating these three HF-containing sections have often proven to be troublesome, complex, and expensive. In addition to these three sections, this HF detergent alkylation unit also uses a series of five product recovery columns to produce a monoalkylbenzene product stream from the alkylation reaction effluent, which contains not only monoalkylbenzene but generally also benzene, paraffins, by-products, and HF. The first of the five product recovery columns is usually called an HF stripper, which strips HF from the alkylation reaction effluent for recycle to the HF alkylation reactor. The second column is generally called a benzene column, which is a distillation column that removes benzene from the HF stripper bottom stream as an overhead stream which is recycled to the HF alkylation reactor. Then, the remaining hydrocarbons flow to a series of three distillation columns: a paraffin column which removes the paraffins as a sidecut for recycle to a paraffin dehydrogenation unit if present, an LAB rerun column which removes LAB from the paraffin column bottom stream and produces an overhead stream containing the LAB product, and a heavy alkylate rerun column that removes heavy alkylate by-products including polyalkylbenzenes.
Changing from HF to a solid catalyst has greatly diminished the utility of this five-column product recovery train in existing HF detergent alkylation units, particularly in two aspects. First, the change to a solid catalyst eliminates the need for HF stripping, thereby rendering the existing HF stripper redundant. Second, the higher benzene/olefin molar ratio (e.g., 20:1 as opposed to 8:1) in the alkylation reactor more than doubles the flow of benzene to the existing benzene column, thereby flooding the existing benzene column. Too small for the higher recycle benzene flow, the existing benzene column must be replaced or supplemented with an entirely new benzene column, which greatly increases the capital cost of converting from HF to solid catalyst. But, even if a new benzene column was not needed, the operating costs of the now-converted solid catalyst unit would be much higher, because of the additional cost of the energy required to distill and condense the larger quantity of excess benzene from the alkylation reaction effluent.
Accordingly, a method is sought that reduces the cost of converting an existing detergent alkylation unit from HF to a solid alkylation catalyst. A process that can reduce the cost of recycling benzene and thereby improve the usefulness of commercially available solid alkylation catalyst will help avoid the need for using HF in detergent alkylation processes.